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When good series go bad, it’s sad, sad, sad.

Not to mention unnecessary. The main reason a good series sags is age.

“The problem with American television is the inability to stop when you’re finished,” says Robert Thompson, director of Syracuse University’s Center for the Study of Popular Television. “Producers here can’t seem to tell a story and then shut up.”

Take “ER,” for example. This NBC medical drama, entering its 10th season Sept. 23, was absolutely stunning its first few seasons. We had never seen anything like it: The pace was breathtaking, the emergencies were eye-popping, (and gruesome), and the characters were intriguing.

But such intensity cannot be sustained for a long period of time. It just can’t. And main characters that are explored in depth eventually run out of depth. Tragically, sometimes they even turn cartoonish.

Dr. Mark Greene was the show’s soul. Greene went through a painful divorce, was attacked and nearly killed in the hospital’s restroom and then developed a deadly brain tumor. By the time he was “killed off” in 2002, his story line was beyond melodrama.

This season is likely to put Dr. John Carter in the same position. He’s gone through a romantic nightmare with Abby Lockhart, thanks to her mentally ill brother and mother; wrestled with a drug problem and had a stillborn child with the lady love he met in Africa. What horrors are left?

“ER” is at least four years past its prime. When a dead baby is the highlight of a season, it’s past time to call it quits.

“The Practice” finally limped off the air last season but should have ended in 2000.

Television networks in other parts of the world rarely keep series alive beyond a year or two. In many European countries, 13 episodes is a long run.

The normal life span of a series, according to Thompson and other TV-watching experts, is five to seven years. Most successful shows start off well, grow in their second season, peak in the third or fourth season and, almost inevitably, begin to fade soon after.